
Note that \(\mathbf {S}^\top \mathbf {S}\)  is diagonal with 1 in positions corresponding to the sampled pixels and 0 elsewhere [1]} (recall that \(\mathbf {S}\)  is the spatial sampling operator). Since the matrix \(\mathbf {E}\mathbf {E}^\top + \rho \mathbf {I}\)  is of size \(L_s \times L_s\) , its inverse can be precomputed and stored.
